Birefringent lens for laser beam delivery

ABSTRACT

An ophthalmic laser system includes a laser engine to generate a beam of femtosecond laser pulses, a laser scanner to scan each laser pulse of the beam in three dimensions according to a scan pattern, and a compound lens comprising a glass lens and a birefringent lens, the compound lens arranged to receive the scanned beam and configured to split each laser pulse of the scanned beam into an ordinary pulse and an extraordinary pulse, producing an ordinary beam comprising ordinary pulses and an extraordinary beam comprising extraordinary pulses. A particular ordinary pulse and a particular extraordinary pulse split from a particular laser pulse are spatially separated in depth along an optical axis of the compound lens, by a distance greater than or equal to 5 μm, and temporally separated by a delay greater than or equal to a pulse duration of the femtosecond laser pulses. An objective is configured to focus the ordinary beam and the extraordinary beam within an ophthalmic target.

FIELD

This present disclosure relates generally to femtosecond laser systems and, more particularly, to femtosecond laser systems for ophthalmic surgical procedures.

BACKGROUND

Surgery with femtosecond ophthalmic lasers entails generating a pulsed laser beam and delivering the laser pulses by a scanning delivery system through a focusing optics to a sequence of focus spots along a scan pattern in a target region of an ophthalmic tissue. Each laser pulse can create a plasma or cavitation bubble in the target tissue at the focus spot of the laser beam when the beam intensity or energy density exceeds a plasma or photodisruption threshold. During surgery, the focus spot of the laser beam is scanned along a three-dimensional scan pattern, creating a sequence of these bubbles to form macroscopic surgical cuts or photodisrupted regions.

Laser-assisted cataract surgery (LACS) requires the application typically a few million laser pulses. The LenSx® femtosecond laser system manufactured by Alcon® is capable of completing a typical LACS procedure which includes capsulotomy and lens fragmentation in about one minute. During such procedures, the patient lays prone beneath the laser system while the eye is immobilized by a patient interface. The accuracy, precision, and patient experience of LACS can be improved by shortening the procedure time. Accordingly, there is a need for simple, inexpensive, and reliable technique to reduce procedure times of laser-assisted ophthalmic surgeries.

SUMMARY

In general, the present disclosure relates to a multifocal laser beam delivery system for femtosecond laser systems. Certain embodiments provide a laser system that includes a laser engine configured to generate a beam of femtosecond laser pulses, a laser scanner configured to scan each laser pulse of the beam in three dimensions according to a scan pattern, and a compound lens (e.g., a doublet lens, triplet lens, etc.) comprising a glass lens and at least one birefringent lens. The compound lens is arranged to receive the scanned beam and configured to split each laser pulse of the scanned beam into an ordinary pulse and an extraordinary pulse, thereby producing an ordinary beam comprising ordinary pulses and an extraordinary beam comprising extraordinary pulses. A particular ordinary pulse and a particular extraordinary pulse split from a particular laser pulse are spatially separated in depth along an optical axis of the compound lens, by a distance greater than or equal to 5 μm, and temporally separated by a temporal delay greater than or equal to a pulse duration of the femtosecond laser pulses. The system includes an objective configured to focus the ordinary beam and the extraordinary beam within an ophthalmic target.

In variants of the disclosed system the scan pattern may be a spiral or raster scan pattern, and the birefringent lens may comprise a birefringent crystal. Embodiments may include a rotation lens mount coupled to the birefringent lens, such that the birefringent lens is rotatable to adjust a relative energy content of the ordinary beam and the extraordinary beam. The birefringent lens may be configured such that rotating the birefringent lens to a first position causes the relative energy content between the ordinary beam and the extraordinary beam to be equally distributed, and rotating the birefringent lens to a second position causes at least 95%, 98%, or 99% of the relative energy to be distributed to the ordinary beam and no more than 5%, 2%, or 1% of the relative energy to be distributed to the extraordinary beam.

Embodiments may include a rotation mount coupled to a halfwave plate positioned between the scanner and the birefringent lens, such that the halfwave plate is rotatable to rotate the polarization of the scanned beam and thereby adjust the relative energy content of the ordinary beam and the extraordinary beam. The halfwave plate may be configured such that rotating the halfwave plate to a first position causes the relative energy content between the ordinary beam and the extraordinary beam to be equally distributed; and rotating the halfwave plate to a second position causes at least 95%, 98%, or 99% of the relative energy to be distributed to the ordinary beam and no more than %, 2%, or 1% of the relative energy to be distributed to the extraordinary beam.

In some variants of the disclosed system the spatial separation in depth of the ordinary and extraordinary pulses along the optical axis is in the range of 5-100 μm, and the delay temporally separating the ordinary and extraordinary pulses is in the range of 100-1000 femtoseconds.

In certain embodiments, the system comprises a laser controller configured to generate a scan pattern based on a rotation position of the birefringent lens or halfwave plate, and control the scanner to scan the beam according to the generated scan pattern. The scan pattern may be a three-dimensional spiral scan pattern, or a raster scan pattern.

In certain embodiments, the laser engine is configured to generate the beam of femtosecond laser pulses with a pulse energy sufficient to cause photodisruption in an ophthalmic target at focus spots of the ordinary beam and the extraordinary beam. The compound lens may be configured so that the spatial and temporal separation between the particular ordinary pulse and particular extraordinary pulse is sufficient to cause a photodisruption at a first depth in the ophthalmic target before causing photodisruption at a second depth in the ophthalmic target, wherein the first depth is greater than the second depth relative to an external surface of the ophthalmic target.

In variants of the disclosed system, the birefringent lens may be made of calcite (CaCO₃), ytrium orthovanadate (YVO₄), lithium niobate (LiNbO₃), barium borate (BaB₂O₄), or other birefringent crystal material.

Embodiments of the disclosure provide one or more advantages and benefits. For example, the z-doubling technique described herein can be used to effectively increase the frequency of a pulsed laser by a factor of 2λ, without increasing the actual scan rate. Accordingly, embodiments provide a simple and reliable optical approach for increasing scanning rate without modifying the laser engine or beam scanner (eliminating potential galvo speed issues, pockels cell issues, etc. attendant to doubling the repetition rate by conventional means). It can be applied and tested in existing machines with minimal modification.

Additionally, embodiments are configured to focus two laser spots in close enough proximity (e.g., ≈25 μm) to improve lens fragmentation efficiency by a factor of more than 2×. Even more, the z-doubling effect of the birefringenent lens can be “turned on” or “turned off” by rotating the plane of polarization of the beam incident on the lens, or rotating the birefringent lens itself. Hence, the relative energy content of the ordinary and extraordinary foci can be easily controlled by mounting the birefringent lens or a halfwave plate on a rotating lens mount. This enables simple reconfiguration of a machine for different procedures (e.g., cataract and refractive procedures), and allows a user to control the direction of propagation of expelled gas during an ophthalmic procedure where z-doubling is employed.

Further, because the shallow focus spot is delayed in time relative to the deeper focus spot, shadowing effects caused by plasma generation will not impede procedures which employ the z-doubling techniques described herein.

These and other benefits will be apparent to one skilled in the art in view of the drawings and specification.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which like reference numerals indicate like features and wherein:

FIG. 1 is a block diagram of a laser system, according to certain embodiments;

FIG. 2 is a schematic of components of delivery optics in a laser system, according to certain embodiments;

FIGS. 3A and 3B are diagram illustrating effects of shadowing caused by a focal spot;

FIG. 4 depicts a method, according to certain embodiments;

FIG. 5 is a schematic of components of delivery optics in a laser system, according to certain embodiments.

One skilled in the art will understand that the drawings, described below, are for illustration purposes only. The drawings are not intended to limit the scope of the claims or disclosure.

DETAILED DESCRIPTION

The following description is presented to enable one skilled in the art to make and use the inventions disclosed herein, and is generally provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be apparent to those skilled in the art, and it is understood that general principles set forth herein may be applied to other embodiments and applications without departing from the spirit and scope of the disclosure. It is further understood that systems, devices, components, and methods described with respect to one embodiment may be combined with features, components, and/or steps described with respect to other embodiments. For the sake of brevity, however, the numerous iterations of these combinations will not be described separately. Thus, the scope of the disclosure is not limited to the embodiments described, but is to be accorded the widest scope consistent with the claims.

In general, the present disclosure relates to an improved multifocal laser beam delivery system for femtosecond laser systems. In certain embodiments, a compound lens (e.g., a doublet or triplet lens) comprising at least one birefringent lens is included in the delivery optics of a femtosecond laser. The compound lens can split the laser energy generated by a laser engine into an ordinary and an extraordinary beam which may be focused “below” each other (separated in the depth direction, along the optical axis of the laser system) so that the laser spot of each beam forms a breakdown cavity in the target (such as ophthalmic tissue to be treated). This may be referred to as “z-doubling.” In this way, the effective laser repetition rate can be doubled without increasing the repetition rate of the laser engine, thereby avoiding technical challenges which accompany doubling the repetition rate of a laser source. Additionally, the compound lens may impart a delay so that the ordinary and extraordinary beams are temporally separated, forming a breakdown cavity at a first depth before forming a breakdown cavity at a second, shallower depth in the target. In this manner, any distortion and shadowing resulting from optical breakdown at the second depth will not interfere with optical breakdowns at the first depth. Without such a delay, the plasma generated at the second (shallower) depth may cast a shadow which interferes with the focal spot at the first (deeper) depth, impeding formation of a breakdown cavity at the first depth. Accordingly, embodiments provide a simple and reliable solution for increasing the effective repetition rate of a femtosecond laser system.

FIG. 1 is a block diagram of an example ophthalmic surgical laser system 100. The system includes a pulsed laser source 110, scanners 120, delivery optics 130, patient interface 140, imaging device 150, and laser controller 160. One skilled in the art will appreciate that system 100 may include additional components which are not shown for the sake of simplicity.

Pulsed laser source 110 (also referred to as laser engine 110) may comprise a laser engine capable of generating a beam 101 of femtosecond laser pulses. In certain variants, laser engine 110 comprises a chirped pulse amplification (CPA) laser which includes an oscillator to generate femtosecond seed pulses, a stretcher to stretch the seed pulses by a factor of 10-1000 to the picosecond range, an amplifier to amplify the pulses, and a compressor to compress the length of the amplified pulses back to the femtosecond range. In certain examples, laser engine 110 may be a cavity-dumped regenerative amplifier laser which includes an oscillator, stretcher-compressor, and optical amplifier. Laser engine 110 may, in certain variants, comprise a bulk laser, fiber laser, or hybrid laser design. In certain variants, the laser pulses generated by laser engine 110 may have a pulse duration in the range of 100-10,000 femtoseconds, 600-5000 femtoseconds, or 600-1000 femtoseconds, a per-pulse energy in the range of 0.1-1000, and a repetition frequency in the 1 kHz-1 MHz range. In various examples, generated laser pulses may have laser pulse parameters which fall within a combination of these range limits, such as a range of pulse duration of 10 fs-10 ps Specific laser parameters for a particular procedure may be selected within such ranges based on patient-specific and/or procedure-specific data. When splitting pulses (e.g., z-doubling) for a procedure such as a cataract procedure, each generated laser pulse has a total energy that is at least two times (2×) the photodisruption threshold of the target tissue, so that each ordinary and extraordinary pulse produced by a compound lens 200 has sufficient energy to independently induce photodisruption at ordinary and extraordinary focal spots.

Scanners 120 includes any suitable combination of x-y scanners and z-scanners (which may include steering mirrors, galvanometers, lenses, servomotors, etc.) which scan the beam 101 to direct focal spots to a series of points in a three-dimensional surgical scan pattern in the target tissue. Each laser pulse can create a plasma or cavitation bubble in an ophthalmic target 103 at a focal spot 102 of beam 101 when the intensity or energy density exceeds a plasma or photodisruption threshold. Scanners 120 thus control where photodisruption occurs in the target, and scan focus spot 102 in three dimensions within target 103 to execute a surgical plan. An x-y scanner scans focus spot 102 of beam 101 in an x-y plane perpendicular to an optical axis of the laser system 100, while a z-scanner scans focus spot 102 of beam 101 in the z-direction to particular depths in target 103 along the optical axis of the laser system 100. Various embodiments may include one or more z-scanners positioned between pulsed laser 100 and an x-y scanner, between the x-y scanner and delivery optics 130, or both. In some embodiments, a z-scanner may additionally or alternatively be located within delivery optics 130. Scanners 120 may be configured to scan beam 101 in any suitable surgical pattern, including spiral patterns, raster patterns, elliptical patterns, circular patterns, spider patterns, etc. The scan patterns may be automatically and/or manually generated by a processor of laser controller 160 based on patient and procedure-specific data and forwarded to scanner 120 via control signals 106.

Delivery optics 130 may include a focusing objective lens, beam expander, birefringent lens (discussed further below), and other lenses to direct, collimate, condition, and/or focus the scanned beam 101 through the patient interface 140 to focus spot 102 in target 103. Components of delivery optics 130 may be slidably or movably mounted on a base so that the weight of the objective lens and other optical components is balanced to limit force applied on patient interface 140 and target 103. Patient interface 140 may include, for example, a one or two-piece transparent applanation lens attached to a mount on delivery optics 130. The mount can provide a stable connection between the patient interface and delivery optics 130. Patient interface 140 may attach to and immobilize a target such as an eye during a laser procedure.

System 100 may additionally include one or more imaging devices 150. In certain embodiments, system 100 includes a surgical microscope, video microscope, digital microscope, ophthalmoscope, and/or camera to receive imaging light 104 and generate live images of the surface of target 103 during a procedure. System 100 may also include enhanced imaging devices to assist in guiding the laser surgery. In one example, imaging device 150 includes an optical coherence tomography (OCT) imaging system to generate depth-resolved images of the inner structure of target 103, such as the location, position, and curvature of the crystalline lens, the anterior and posterior capsules, and the cornea. Imaging device 150 may comprise any suitable SS-OCT, SD-OCT, or TD-OCT system for generating static or live A-scan, B-scan, or 3D OCT images. Imaging devices 150 may comprise x-y and z-scanners to scanning an imaging beam into target 103 (not shown).

Image data 105 generated by imaging device 150 may be provided to a laser controller 160. Laser controller 160 comprises memory (“M”) storing instructions executable by a processor (“P”) to control pulsed laser source 110, scanners 120, delivery optics 130, and/or imaging devices 150. Typically, the processor of laser controller 160 comprises one or more CPUs (such as those manufactured by Intel, AMD, and others), microprocessors, field-programmable gate arrays (FPGAs), application-specific integrated circuits (ASICs), digital-signal processors (DSPs), or system-on-chip (SoC) processors communicatively coupled to memory. The memory may comprise a non-transitory computer-readable medium, and may include volatile or non-volatile memory including, magnetic media, optical media, random access memory (RAM), read-only memory (ROM), removable media, or analogous components. The memory may store software instructions executable by the processor to generate control signals 106 that control the operation of pulsed laser source 110, scanners 120, delivery optics 130, and imaging device 150.

For example, laser controller 160 may generate signals 106 to control parameters of beam 101 generated by pulsed laser source 110, such as a repetition rate, pulse length, and pulse energy. Laser controller 160 may also generate signals 106 to actuate individual components of scanners 120 in order to direct focus spot 102 according to a surgical scan pattern. Such scan patterns may be any suitable 2D or 3D shape or pattern, including spiral, raster, circular, elliptical, cylindrical, or spider patterns. For example, the scan pattern may be a three-dimensional spiral scan pattern in which the laser beam is focused to particular spots in an x-y plane in a spiral sequence at various z-depths within the cylinder. In certain examples, laser controller 160 generates scan patterns to create a capsulotomy cut in the anterior capsule of a target 103 and fragment a crystalline lens of target 103 prior to extraction. Laser controller 160 may generate control signals based on image data 105 (e.g., OCT images) obtained intraoperatively during a surgical procedure. In this manner, system 100 can perform image-guided ophthalmic surgery based on real-time feedback.

It is noted that, to the extent used in the claims below, the terms “processor,” and “memory” refer to structures specially configured for a femtosecond laser system, as known to those skilled in the art, and those terms should not be construed or interpreted as means-plus-function elements.

FIG. 2 illustrates aspects of delivery optics 130 according to certain embodiments. Delivery optics 130 may include compound lens 200 comprising lens 210 and at least one birefringent lens 220, beam expander 230, and objective 240. In the illustrated example, compound lens 200 comprises a doublet lens. In other embodiments, compound lens 200 may comprise a triplet lens, or comprise additional lenses. Pulsed beam 101 generated by laser source 110 and scanned by scanners 120 propagates along optical axis 250 to compound lens 200, which splits the beam into an ordinary beam 101 o and extraordinary beam 101 e having different wavefront curvatures and temporal delays.

It is noted that, in various embodiments, the arrangement of optical elements in delivery optics 130 may vary. For example, compound lens 200 could be placed between beam expander 230 and objective 240 in certain variants, or between multiple lenses of beam expander 230.

Ordinary and extraordinary beams 101 o and 101 e arrive at beam expander 230, which comprises one or more lenses which increase the diameter of beams 101 o and 101 e. Beam expander 230 may be a movable or fixed block of lenses and in some embodiments may include a z-scanner. In certain embodiments, positioning lens 200 at different locations within beam expander 230 can determine the relative separation of ordinary and extraordinary focus spots 250 o and 250 e without affecting the temporal delay between them, as discussed with respect to FIG. 5 below.

Expanded beams 101 o and 101 e arrive at objective 240. Objective 240 may comprise a focusing objective with one or more lenses having a total focal length F. Objective 240 focuses beams 101 o and 101 e to focus spots 250 o and 250 e that are spatially separated in depth by a distance Δ along optical axis 250 and temporally separated by a delay time. The objective 240 can focus the expanded laser beam into a high numerical aperture (high NA) focused beam and deliver it to the target region through patient interface (PI) 140. Generating a high NA beam can ensure that the ordinary and extraordinary beams cause photodisruption only at the intended z-depth or longitudinal (x-y) coordinate, thus avoiding collateral damage posterior or anterior relative to the target region. In some embodiments, NA can be in the 0.10-0.45 range. In some others, it can be in the 0.25-0.35 range.

As noted above, compound lens 200 comprises complimentary lens 210 and at least one birefringent lens 220. In certain examples, lens 210 comprises a glass, such as Schott BK7. In other examples, lens 210 may comprise another material with suitable optical and mechanical characteristics. Birefringent lens 220 may comprise birefringent crystal having an ordinary refractive index n_(o) and an extraordinary refractive index n_(e). Variants of birefringent lens 220 may be made from calcite (CaCO₃), ytrium orthovanadate (YVO₄), lithium niobate (LiNbO₃), and barium borate (BaB₂O₄), or other birefringent materials.

As noted above, in certain embodiments compound lens 200 may be differently positioned within the delivery optics. In one example, beam expander 230 may comprise at least two lenses 230 a and 230 b, as shown in FIG. 5, and compound lens 200 may be adjustably positioned between lenses 230 a and 230 b. In this manner, the z-separation (distance) between ordinary and extraordinary focus spots 250 o and 250 e can be continuously varied. In different arrangements, the magnitude and sign (positive or negative) of a a z-separation (Δ shown in FIG. 2) can be controlled in this manner. For example, when compound lens 200 is moved (e.g., by an actuator) to focus point 500 of beam expander 230, a z-separation (Δ shown in FIG. 2) between the ordinary and extraordinary focus spots 250 o and 250 e will be zero. If compound lens 200 is moved (e.g., by the actuator) from point 500 toward the source of the laser beam (to the left of focus spot 500 shown in FIG. 5—as illustrated), the ordinary spot 250 o will be located optically upstream (in FIG. 5, to the left) of extraordinary spot 250 e. Moving lens 200 to the other side of focus spot 500 reverses this arrangement. Thus, if compound lens 200 is moved (e.g., by the actuator) from point 500 away from the source of the laser beam (in FIG. 5, to the right of focus spot 500), the extraordinary spot 250 e will be located optically upstream (in FIG. 5, to the left) of ordinary spot 250 o. The temporal delay between spots 250 o and 250 e is not affected by the position of lens 200 within beam expander 230.

In the example shown in FIG. 2, lens 210 has a negative power and birefringent lens 220 has a positive power. Although the disclosure is not limited to the arrangement shown (either component lens 210 or 220 may be negative, positive, or neither, according to particular design goals), the design of lens 210 and birefringent lens 220 may be optimized to split beam 101 into an ordinary beam 101 o and extraordinary beam 101 e with particular characteristics. For example, the focal lengths and refractive indices of lens 210 and birefringent lens 220 may be selected to impart particular wavefront curvature and temporal delay to the ordinary and/or extraordinary beams. In certain variants, lens 210 may comprise a material with a relatively low refractive index and low dispersion, and may be fabricated with a positive power to complement the refractive indices n_(o), n_(e) and a negative power of birefringent lens 220, in order to split beam 101 into ordinary and extraordinary beams 101 o and 101 e which are spatially separated in depth along optical axis 250 and temporally separated by a delay.

The relative spatial separation in depth Δ of ordinary beam spot 250 o and extraordinary beam focus spot 250 e shown in FIG. 2 may be may be approximated as follows: Δ=[(n _(o) −n _(e))/(n _(e)−1)]*F ²/(M ² /f ₀)

Where:

n_(o) is an ordinary refractive index of birefringent lens 220;

n_(e) is an extraordinary refractive index of birefringent lens 220;

F is the focal length of objective 240;

M is the magnification of beam expander 230; and

f₀ is the focal length of lens 210.

Hence, in various embodiments, optical characteristics of the components shown in FIG. 2 may be optimized to separate focus spots 250 e and 250 o by a distance of, for example, 20 μm, 25 μm, or 30 μm. Other variants may separate focus spots 250 e and 250 o by distances in the range of 10-40 μm, 15-35 μm, 20-30 μm, or other ranges.

In addition to separation in depth along the optical axis 250, it can be beneficial to impart a temporal separation between the focus spots to eliminate problems which may arise from “shadowing.” FIG. 3A illustrates how “shadowing” resulting from focusing an ordinary spot 250 o at a “shallow” depth can interfere with focusing an extraordinary spot 250 e at a greater depth along optical axis 250.

Generally speaking, photodisruption occurs when a laser pulse is focused in ophthalmic tissue with sufficient energy to form plasma in the area of the focal spot. Rapid expansion of the plasma is followed by emission of a shock wave and formation of a cavitation bubble. As a result, tissue within the focal volume is destroyed. Multiple focal spots applied in a scan pattern can be used to create precise incisions in ophthalmic tissue. However, the plasma formed at the focal spot is not transparent. As a result, if ordinary beam 101 o and extraordinary beam 101 e are simultaneously focused to spots 250 o and 250 e, as shown in FIG. 3A, shadowing 300 results when plasma generated by spot 250 o blocks part of extraordinary beam 101 e and prevents or impedes photodisruption at focus spot 250 e.

Such shadowing effects can be avoided by configuring birefringent lens 220 to introduce a temporal delay between the ordinary and extraordinary beams, in addition to the spatial separation in depth discussed above. In certain examples, birefringent lens 220 is designed to delay pulses of one or the other of ordinary or extraordinary beam by at least the duration of each pulse generated by the pulsed laser source 110. As a result, plasma will develop at a spot of lesser depth (e.g., spot 250 o) only after the spot at the greater depth (e.g., spot 250 e) has been delivered, as shown in FIG. 3B. In this manner, shadowing caused by plasma at focus spot 250 o does not affect photodisruption at focus spot 250 e. Variants of birefringent lens 220 may be designed to induce a relative delay between ordinary and extraordinary beam pulses of at least 300, 500, 800, 1000, 1200, 1500, or 2000 femtoseconds, or in the range of 100-10,000 femtoseconds.

Returning to FIG. 2, each pulse of pulsed beam 101 is split by compound lens 200 to generate an ordinary beam 101 o having an ordinary wavefront 201 o and ordinary beam focus spot 250 o, and an extraordinary beam 101 e having an extraordinary wavefront 201 e and an extraordinary beam focus spot 250 e (disregarding Fresnel reflection). The relative energy content of the ordinary and extraordinary foci 250 o and 250 e may be described as follows: E _(e) =E*cos²(ϕ) E _(o) =E*sin²(ϕ)

Where:

E_(e) is the energy of the extraordinary beam 101 e;

E_(o) is the energy of the ordinary beam 101 o;

ϕ is the angle of polarization of beam 101 with respect to the optical axis of the birefringent material;

and E is the total energy of beam 101 before it arrives at compound lens 200.

Thus, the relative energy content of the ordinary and extraordinary beams 101 o and 101 e can be controlled by rotating the angle of polarization of beam 101. For example, when the plane of polarization of beam 101 is rotated to ϕ=45°, the relative energy content of the ordinary and extraordinary foci produced by splitting will be equal, such that the ordinary beam 101 o and extraordinary beam 101 e each have approximately 50% of the total energy of beam 101 when it arrives at compound lens 200. Similarly, when the plane of polarization of beam 101 is rotated to ϕ=0°, the relative energy content of the ordinary or extraordinary foci will approach a 100% to 0% split, effectively eliminating the multiplexing effect of birefringence (only one of the ordinary or extraordinary focal spot will have sufficient pulse energy to cause photodisruption).

The angle of polarization ϕ may be adjusted in various ways. For example, birefringent lens 220 or compound lens 200 may be mounted into a rotating lens mount such that rotating birefringent lens 220 independently or compound lens 200 as a unit within the mount changes the angle of polarization of beam 101. Additionally or alternatively, the angle of polarization of beam 101 may be rotated independently of birefringent lens 220 by including a half (λ/2) waveplate (not shown in FIG. 2) positioned in the path of beam 101 before it impinges on birefringent lens 220. For example, a half wave crystal quartz waveplate may be coupled to a rotating lens mount arranged between scanner 120 and compound lens 220, such that rotating the waveplate changes the angle polarization of beam 101 before it is received by compound lens 220.

Adjusting the angle of polarization ϕ may be particularly useful in multipurpose laser systems capable of performing different operations, such as ophthalmic laser systems for both anterior segment (e.g., cataract) and corneal (e.g., refractive) surgeries. In a cataract procedure (e.g., capsulorhexis or fragmentation), laser incisions are made in and around the crystalline lens at depths between 4-12 mm relative to an anterior surface eye. In contrast, a refractive procedure (e.g., e.g., flap cutting, lenticule removal, etc.) entails making laser incisions in the cornea at relatively shallow depths between 0.01-1 mm relative to an anterior surface eye. Accordingly, in preparation for such a cataract procedure, the angle of polarization of beam 101 may be adjusted to ϕ=45° to split the energy of the pulsed beam equally between ordinary and extraordinary beams spatially separated in depth along an optical axis (z-axis) of the laser system. This effectively doubles the repetition rate of the laser at depth, reducing cataract procedure times without increasing the actual repetition rate of the laser engine. Prior to a refractive procedure, the angle of polarization of beam 101 may be adjusted to ϕ=0° so that little or no energy of the pulsed beam is split to either the ordinary and extraordinary beam (depending on the particular implementation). Rotation to different positions may be performed manually by a system operator or by signals sent by laser controller 160 in response to user commands or as part of an automatic configuration sequence for system 100. Accordingly, embodiments of system 100 may automatically set the angle of polarization of beam 101 (by rotating birefringent lens 220 or a halfwave plate) to a cataract position (ϕ=45°) during calibration prior to a cataract procedure, and alternatively to a refractive position (ϕ=0°) curing calibration prior to a refractive procedure.

It is further noted that adjusting the relative energy content of the ordinary and extraordinary beams may be useful to influence the interaction of multiple, near-photodisruption-induced cavitation bubbles caused by spots 250 o and 250 e. In general, when two cavitation bubbles are sufficiently close in space and time to interact, a strong jet of bubbles is formed in the vertical direction (along optical axis 250). The direction of the jet propagation (i.e., the direction along optical axis 250) can be precisely controlled by the relative energy between the ordinary and extraordinary focus spots 250 o and 250 e. The physics of cavitation bubble interaction is such that the jet is formed toward the smaller bubble. Thus distributing less energy for the extraordinary focus spot 250 e depicted in FIG. 2, the jet will be directed away from the source of beam 101. In this way, unwanted interaction of the jet with incident laser pulses 250 o, 250 e can be eliminated. The polarization angle of beam 101 may thus be rotated in the manner described to control the interaction of cavitation bubbles during a procedure, such as a lens fragmentation or capsulorhexis.

Returning to FIG. 2, in an example embodiment lens 210 comprises BK7 and has a focal length f₀=1425.6 mm. Birefringent lens 220 is made of calcite with an ordinary refractive index n_(o)=1.658, an extraordinary refractive index n_(e)=1.486, and a central thickness of 1.744 mm. Lens 210 and birefringent lens 220 are designed such that compound lens 200 has zero optical power for extraordinary beam 101 e and +0.25 diopter power for the ordinary beam 101 o. In such a configuration, passing beam 101 through compound lens 200 in which lens 210 is BK7 and birefringent lens 220 is calcite splits the beam into an ordinary beam 101 o having a curved wavefront with a 4 meter radius of curvature, and an extraordinary beam 101 e having a flat wavefront. Moreover, the curved ordinary wavefront 201 o is temporally delayed relative to the flat extraordinary wavefront 201 e. In calcite, n_(o)<n_(e), and the ordinary beam 101 o will be delayed with respect to the extraordinary beam 101 e by approximately 1000 femtoseconds. Beams 101 o and 101 e propagate to a 5× beam expander 230 which increases the radius of curvature of ordinary beam wavefront 201 o from 4 meters to 100 meters, while the radius of curvature of extraordinary beam wavefront 201 e remains flat. Beams 101 o and 101 e are then focused by objective 250 having a focal length F=50 mm. Accordingly, ordinary beam focus spot 250 o and extraordinary beam focus spot 250 e will be separated by distance Δ: Δ=[(n _(o) −n _(e))/(n _(e)−1)]*F ²/(M ² /f ₀): Δ=(0.172/0.486)*50²/(5²/1415.6)=25 μm

In this example, delivery of ordinary beam 101 o is delayed with respect to the extraordinary beam wavefront 101 e by approximately 1000 femtoseconds. Thus, in this example, an extraordinary pulse is delivered before the ordinary pulse to a given x-y position at a particular z-depth. That is, ordinary pulse is focused at approximately the same x-y position as the extraordinary pulse, but at a z-depth which is 25 μm closer to the anterior surface of the target, and approximately 1000 femtoseconds after delivery of the extraordinary pulse. As a result, plasma will develop at the ordinary beam focus spot 250 o only after the extraordinary beam focus spot 250 e has been delivered, avoiding adverse effects of shadowing. Temporal delay between the ordinary and extraordinary breakdowns can also be achieved by inserting a properly oriented birefringent slab of proper thickness into the beam before the compound lens 200.

Such a configuration (and similar variants) may be well-suited for ophthalmic applications, as it is known that photodisruption-induced cavitation bubbles generated in succession (e.g., within 5000 femtoseconds) at close distances (e.g., within 20-30 μm) interact with each other, increasing cutting efficiency in optical tissue. In particular, placing two focal spots approximately 25 μm apart in ophthalmic tissue in less than 5000 femtoseconds may produce a cutting effect which exceeds the effect of two spots of comparable energy applied further apart in time or distance. Accordingly, components depicted in FIG. 2 may be optimized to generate ordinary beam focus spot 250 o and extraordinary beam focus spot 250 e which are spatially separated in depth along optical axis 250 by a distance of approximately 25 μm, or in the range of 20-30 μm, and temporally separated by a delay greater than or equal to a pulse duration of the femtosecond laser pulses (e.g., 100-10,000 femtoseconds).

It is noted that parameters described in the examples above are provided to illustrate a particular embodiment and do not limit scope of disclosure. One skilled in the art will appreciate that various other configurations (material combinations, lens powers, etc.) will be suitable to optimize performance for a particular application. Although application ophthalmic procedures such lens fragmentation and capsulorhexis are discussed in detail, it is contemplated that embodiments may be useful for other ophthalmic procedures, including corneal procedures, vitreo-retinal procedures, or other procedures. Embodiments may also be useful outside the ophthalmic context, to configure a femtosecond laser to generate two focal spots spatially separated in depth along an optical axis and temporally separated by a delay.

FIG. 4 depicts a method 400 for performing a surgical procedure using laser system 100, according to certain embodiments. At step 402, a relative energy distribution between an ordinary and extraordinary beam may be set to a cataract setting. As described above with respect to FIG. 2, the angle of polarization of beam 101 may be adjusted to ϕ=45° (by rotating birefringent lens 220 or a halfwave plate) to split the energy of a pulsed beam equally between ordinary and extraordinary beams spatially separated in depth along an optical axis (z-axis) of the laser system. This effectively doubles the repetition rate of the laser at depth, reducing cataract procedure times without increasing the actual repetition rate of the laser engine. By using the Z doubling technique the laser scanners can also run at half the speed otherwise required to achieve an analogous surgical effect. Alternatively, the relative energy distribution may be set to a corneal setting such that only one of the ordinary and extraordinary foci has sufficient energy to cause photodisruption. This is useful for procedures in which incision depths are relatively small. The energy distribution setting may be based on procedure identification and specifications input by a system operator via a user interface.

In certain embodiments, a system operator may initiate a system calibration or configuration process for system 100 to automatically set the angle of polarization of beam 101 to a desired setting. For example, laser controller 160 may receive signals indicating a cataract procedure (e.g., capsulotomy, lens fragmentation, etc.) or corneal procedure (flap, lenticule, tunnel, ring, etc.) is to be performed and in turn send signals to an actuator configured to rotate birefringent lens 220 or a halfwave plate to the requisite position.

At step 404, laser controller 160 generates a scan pattern consistent with the setting selected at step 402. In certain examples, laser controller 160 may analyze biometric data, an OCT image of a target, or other information and, based on the information and selected setting, generate a scan pattern for a surgical procedure that accounts for the photodisruptive effect of the ordinary and extraordinary focus spots. The scan pattern may be, for example a scan pattern for a lens fragmentation procedure, a capsulotomy, a LASIK incision, a lenticular incision, etc. In some examples, the scan pattern may be a spiral pattern, spider pattern, elliptical pattern, or any other suitable pattern.

At step 406, a beam of femtosecond laser pulses is generated by laser engine 110. Each laser pulse may have a total energy that is at least 2× the photodisruption threshold of the target ophthalmic tissue, so that each ordinary and extraordinary pulse produced at step 410 has sufficient energy to induce photodisruption independently.

At step 408 the generated pulses are scanned in three dimensions by a laser scanner 120 according to the generated scan pattern. At step 410, each pulse of the generated laser beam is split into an ordinary and extraordinary pulse with approximately equal energy (e.g., a 50/50 energy split, ±3%). A compound (e.g., doublet, triplet, etc.) lens 200 comprising a lens 120 and a birefringent lens 220 may be used as described with respect to FIGS. 2, 3A and 3B. As a result, compound lens 200 generates an ordinary beam comprising ordinary pulses and an extraordinary beam comprising extraordinary pulses. A particular ordinary pulse and a particular extraordinary pulse resulting from splitting a particular laser pulse may be spatially separated in depth, along an optical axis of the compound lens 200, by a distance greater than or equal to 5 μm (e.g., 15-35 μm), and may be temporally separated by a delay greater than or equal to a pulse duration of the laser pulse (e.g., 100-10,000 femtoseconds).

At step 412, the ordinary and extraordinary beams are delivered into the target to perform the cataract procedure. Delivery may entail passing the beams through a beam expander 230 and objective 240 as shown in FIG. 2. As a result of the pulse energy, spatial separation, and temporal separation, an extraordinary pulse (of the extraordinary beam) causes photodisruption at a first depth in the target tissue before the ordinary pulse (of the ordinary beam) causes photodisruption at a second, greater depth in the target tissue (depth being relative to an external surface of the ophthalmic target). The cataract procedure may be, for example, a lens fragmentation and/or capsulorhexis procedure.

At step 414, a relative energy distribution between an ordinary and extraordinary beam may be set to a refractive setting. As described above with respect to FIG. 2, the angle of polarization of beam 101 may be adjusted to ϕ=0° (by rotating birefringent lens 220 or a halfwave plate) to minimize or eliminate the split of energy between ordinary and extraordinary beams. In certain embodiments, the ϕ=0° polarization position distribute at least 95%, 98%, or 99% of the relative energy to either the ordinary or extraordinary beam.

In certain embodiments, a system operator may initiate a system calibration or configuration process for system 100 to automatically set the angle of polarization of beam 101 to a refractive setting or any other suitable procedure setting. For example, laser controller 160 may receive signals indicating a refractive procedure is to be performed and in turn send signals to an actuator configured to rotate birefringent lens 220 or a halfwave plate to the requisite position for a corneal procedure.

At step 416, laser controller 160 generates a scan pattern based on the refractive setting selected above. In this regard, laser controller 160 may generate a refractive scan pattern that will not utilize photodisruptive effects of dual focus spots. The scan pattern may, for example, be configured to define a corneal flap, lenticule, tunnel, etc. At steps 418-422, a beam of femtosecond laser pulses is generated, scanned, and delivered to the cornea to perform a laser-assisted refractive procedure.

One skilled in the art will appreciate that various steps of method 400 may be optional, and may be performed in different order. For example, steps 402-412 may be performed without steps 414-422. Likewise, steps 414-422 may be performed without steps 402-412.

Accordingly, embodiments of the disclosure utilize a compound lens with a birefringent lens component, together with other optical elements, to split the energy of a femtosecond laser beam pulse into an ordinary and extraordinary beam, each with an energy sufficient to induce photodisruption in ophthalmic tissue. The two beams may be focused below each other in a target with a spatial separation in the range of a few tens of a micron and a temporal delay commensurate with the pulse length of the beams. Thus, each laser spot can form a breakdown cavity in tissue under treatment. In this way, the effective laser repetition rate of a laser system can be doubled without increasing the repetition rate of the laser engine.

The techniques and methods of the disclosure thus provide numerous advantages and benefits. For example, a birefringent double lens such as compound lens 200 can be implemented with an existing laser engine and scanners at relatively low cost. Further, embodiments may effectively double the efficiency of a laser source without any increase of the actual repetition rate of the laser engine, thereby avoiding the technical challenges associated with a 100% increase in repetition rate (both in the laser source and scanners). As a result, embodiments of the disclosure provide an inexpensive, reliable optical improvement that can potentially reduce procedure times of ophthalmic procedures by 50% or more, using existing laser equipment at comparatively low cost.

Moreover, because embodiments are configured to split a beam into ordinary and extraordinary pulses separated in depth, they are suitable for use in processes which use scan patterns (e.g., spiral, circular, or elliptic cylinder patterns) that entail cutting at various z-depths (along the optical axis) along lines in the x-y dimension (perpendicular to the optical axis). In contrast, prior multiplexing solutions split a beam into two spots to different points in an x-y plane, but at the same z-depth, and are were thus unsuitable for spiral scan patterns employed by commercial ophthalmic laser systems, including LenSx®.

Additionally, embodiments of the disclosure allow the beam-splitting functionality to be selectively enabled or disabled by rotating an optical component (e.g. the birefringent lens or a halfwave plate) manually or automatically by an actuator under the control of a laser controller. Accordingly, various embodiments can be easily and quickly configured to different modes, such as a cataract mode and a refractive mode, which enable or disable splitting of beams into ordinary and extraordinary beams. These and other benefits will be apparent to one skilled in the art in view of the specification, figures, and claims.

While several of the examples discussed above are described in terms of ophthalmic applications, such as cataract surgery, capsulotomy or corneal procedures, implementations of system 100 can be used in a remarkably wide range of applications, which include a wide variety of ophthalmic procedures, such as retinal and corneal surgery, as well as dermatological and dental applications, different surgical applications, and various material machining applications, which shape a piece of material with laser photodisruption or some other laser aided process.

Accordingly, the foregoing descriptions of various embodiments have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the variants disclosed. Modifications and variations will be apparent to practitioners skilled in the art, and the present disclosure is intended to encompass such modifications and variations. 

The invention claimed is:
 1. An ophthalmic laser system, comprising: a laser engine configured to generate a beam of femtosecond laser pulses; a laser scanner configured to scan each laser pulse of the beam in three dimensions according to a scan pattern; a compound lens comprising a glass lens and at least one birefringent lens, the compound lens arranged to receive the scanned beam and configured to split each laser pulse of the scanned beam into an ordinary pulse and an extraordinary pulse, thereby producing an ordinary beam comprising ordinary pulses and an extraordinary beam comprising extraordinary pulses, wherein a particular ordinary pulse and a particular extraordinary pulse split from a particular laser pulse are spatially separated in depth along an optical axis of the compound lens, by a distance greater than or equal to 5 μm, and temporally separated by a delay greater than or equal to a pulse duration of the femtosecond laser pulses; and an objective configured to focus the ordinary beam and the extraordinary beam within an ophthalmic target.
 2. The system of claim 1, wherein the scan pattern comprises a spiral scan pattern.
 3. The system of claim 1, wherein the birefringent lens comprises a birefringent crystal.
 4. The system of claim 3, further comprising: a rotation lens mount coupled to the birefringent lens; and wherein the birefringent lens is rotatable to adjust a relative energy content of the ordinary beam and the extraordinary beam.
 5. The system of claim 4, wherein the birefringent lens is configured such that: rotating the birefringent lens to a first position causes the relative energy content between the ordinary beam and the extraordinary beam to be equally distributed; and rotating the birefringent lens to a second position causes at least 95% of the relative energy to be distributed to the ordinary beam and no more than 5% of the relative energy to be distributed to the extraordinary beam.
 6. The system of claim 4, further comprising a laser controller configured to: generate a scan pattern based on a rotation position of the birefringent lens; and control the scanner to scan the beam according to the generated scan pattern.
 7. The system of claim 3, further comprising: a rotation mount coupled to a halfwave plate positioned before the scanner and the birefringent lens; and wherein the halfwave plate is rotatable to change an orientation of polarization of the scanned beam and thereby adjust the relative energy content of the ordinary beam and the extraordinary beam.
 8. The system of claim 7, wherein the halfwave plate is configured such that: rotating the halfwave plate to a first position causes the relative energy content between the ordinary beam and the extraordinary beam to be equally distributed; and rotating the halfwave plate to a second position causes at least 95% of the relative energy to be distributed to the ordinary beam and no more than 5% of the relative energy to be distributed to the extraordinary beam.
 9. The system of claim 3, wherein: the spatial separation in depth of the ordinary and extraordinary pulses along the optical axis is in the range of 5-100 μm; and the delay temporally separating the ordinary and extraordinary pulses is in the range of 100-10,000 femtoseconds.
 10. The system of claim 3, wherein the laser engine is configured to generate the beam of femtosecond laser pulses with a pulse energy sufficient to cause photodisruption in an ophthalmic target at focus spots of the ordinary beam and the extraordinary beam.
 11. The system of claim 10, wherein the compound lens is configured so that the spatial and temporal separation between the particular ordinary pulse and particular extraordinary pulse is sufficient to cause a photodisruption at a first depth in the ophthalmic target before causing photodisruption at a second depth in the ophthalmic target, wherein the first depth is greater than the second depth relative to an external surface of the ophthalmic target.
 12. The system of claim 3, wherein the birefringent lens comprises at least one of calcite (CaCO₃), ytrium orthovanadate (YVO₄), lithium niobate (LiNbO₃), or barium borate (BaB₂O₄).
 13. A method of performing ophthalmic surgery, comprising: generating a beam of femtosecond laser pulses; scanning each laser pulse of the generated beam in three dimensions according to a scan pattern; splitting, with a compound lens comprising a glass lens and at least one birefringent lens, each laser pulse of the scanned beam into an ordinary pulse and an extraordinary pulse, thereby generating an ordinary beam comprising ordinary pulses and an extraordinary beam comprising extraordinary pulses; wherein a particular ordinary pulse and a particular extraordinary pulse resulting from splitting a particular laser pulse are spatially separated in depth, along an optical axis of the compound lens, by a distance greater than or equal to 5 μm, and temporally separated by a delay greater than or equal to a pulse duration of the laser pulse; and delivering the ordinary beam and the extraordinary beam into an ophthalmic target according to the scan pattern such that the particular extraordinary pulse causes photodisruption at a first depth in the target tissue before the particular ordinary pulse causes photodisruption at a second depth in the target tissue, wherein the first depth is greater than the second depth relative to an external surface of the ophthalmic target.
 14. The method of claim 13, wherein the scan pattern comprises a spiral scan pattern.
 15. The method of claim 13, wherein the birefringent lens comprises a birefringent crystal.
 16. The method of claim 15, further comprising: rotating the birefringent lens or a polarization of the generated beam to adjust a relative energy content of the ordinary beam and the extraordinary beam.
 17. The method of claim 16, further comprising: rotating the birefringent lens or the polarization of the generated beam to a ϕ=45° polarization to equally distribute the relative energy content between the ordinary beam and the extraordinary beam prior to a cataract procedure.
 18. The method of claim 16, further comprising: rotating the birefringent lens or the polarization of the generated beam to a ϕ=0° polarization to distribute at least 95% of the relative energy to the ordinary beam and no more than 5% of the relative energy to the extraordinary beam prior to a corneal procedure.
 19. The method of claim 16, further comprising: generating the scan pattern based on a rotation position of the birefringent lens; and controlling a scanner to scan the beam according to the generated scan pattern, wherein the scan pattern comprises a spiral scan pattern.
 20. The method of claim 15, wherein: the spatial separation in depth of the ordinary and extraordinary pulses along the optical axis is in the range of 5 μm-100 μm; and the delay temporally separating the particular ordinary and extraordinary pulses is in the range of 100-10,000 femtoseconds. 